1. Field of the Invention
This invention generally relates to surface inspection systems and, more specifically, to an apparatus and method for inspecting a surface using line-illumination imaging techniques.
2. Description of the Related Art
The need to inspect surfaces of objects is prevalent in many situations and for a variety of purposes. For example, it is generally advantageous to inspect surfaces of semiconductor wafers for defects and/or examination of features. Some surface inspection tools employ systems for illuminating a region of a surface, such as a semiconductor wafer. In some systems, the illumination beam may be specifically configured to illuminate a line-shaped region of the surface to be inspected. In such cases, the surface inspection system may further include systems, such as charge coupled devices, for imaging the illuminated line using light sensitive elements (referred to herein as detector elements) to collect photons from the illuminated line. After given period of time, the electrons generated from the photons are converted to a voltage and the measured voltage is passed through an analog-to-digital converter to form a digital image.
In some embodiments, a width of an image of an illumination line on a CCD may be narrower than the pixel size of a detector element along a direction perpendicular to the illuminated line (referred to herein as the “x-direction”), assuming the detector elements are oriented parallel to the illuminated line. A consequence of such a scenario is that the optical resolution of the image along the x-direction is dependent upon the width of the illumination line on the surface being inspected rather than the optical spot size of the collection system. Such a state of affairs may be referred to herein as “one-dimensional (1-D) imaging” since only one dimension (i.e., the dimension parallel to illuminated line, referred to herein as the “y-direction”) of the image is dependent upon the optical spot size of the collection system.
For the reasons noted above, the width of an illumination line may affect the sensitivity of a surface inspection tool in the x-direction and optical spot size of the collection system may affect the sensitivity of the tool in the y-direction. Objects which may be particularly susceptible to surface inspection tool sensitivity are semiconductor wafers. In particular, as feature sizes of semiconductor wafers continue to shrink, more stringent requirements on the sensitivity of wafer inspection instruments are typically imposed. In order to detect relatively small defects and/or features, it may be desirable to minimize the optical spot size of a collection system (to increase sensitivity of a tool in the y-direction) and/or minimize the width of an illumination line on surfaces to be inspected (to increase sensitivity of the tool in the x-direction).
In some cases, however, the width of an illumination line may be limited by the capabilities of the surface inspection tool. For example, some surface inspection tools (e.g., a tool using a 266 nm or greater wavelength laser) are not capable of illuminating lines having width dimensions less than 1.0 micrometer. In order to obviate such a limitation, it would be advantageous to develop a surface inspection system which is configured to image an illuminated line which is independent of the width of the illumination line. In other words, it would be beneficial to develop a surface inspection system based upon two dimensional imaging. In this manner, sensitivity of the surface inspection system in both the x and y directions may be dependent on the optical resolution of the collection system and increasing the sensitivity may be accomplished by reducing the optical spot size of its collection system.